Implement MouseEvent buttons attribute.

Source/platform/PlatformWheelEvent.h

Index: Source/platform/PlatformWheelEvent.h
diff --git a/Source/platform/PlatformWheelEvent.h b/Source/platform/PlatformWheelEvent.h
index 7eebb25f43bd2eff3ea4270297a2f42631aec15b..ce94e734b3c179fd9c89e111f31955316172b33b 100644
--- a/Source/platform/PlatformWheelEvent.h
+++ b/Source/platform/PlatformWheelEvent.h
@@ -104,6 +104,20 @@ public:
 
     bool hasPreciseScrollingDeltas() const { return m_hasPreciseScrollingDeltas; }
     void setHasPreciseScrollingDeltas(bool b) { m_hasPreciseScrollingDeltas = b; }
+
+    unsigned short buttons() const
+    {
+        unsigned short value = 0;
+
+        if (m_modifiers & LeftButtonDown)
+            value |= 1;
+        if (m_modifiers & RightButtonDown)
+            value |= 2;
+        if (m_modifiers & MiddleButtonDown)
+            value |= 4;
+
+        return value;
+    }
 #if OS(MACOSX)
     PlatformWheelEventPhase phase() const { return m_phase; }
     PlatformWheelEventPhase momentumPhase() const { return m_momentumPhase; }